Recently, with the increase in public awareness of environmental issues such as global warming and others, expectations for solar power generation have been much raised in view of the cleanness thereof and freeness from environmental pollution. A solar cell constitutes the main part of a solar power generation system which directly converts the energy of sunlight into electricity. Regarding the configuration of the solar cell, multiple solar cell devices (simply referred to as cells) are connected in series or in parallel and are packaged variously for protecting the cells to give individual units. The unit installed in the package is referred to as a solar cell module, and, in general, this is so designed that the face thereof to be exposed to sunlight is covered with a transparent substrate as an upper protective material (glass or resin sheet—hereinafter this may be expressed as a front sheet), the space is filled up with an encapsulant (encapsulant resin layer) of a thermoplastic resin (for example, ethylene-vinyl acetate copolymer or polyethylene polymer), and the back face is protected with a back encapsulant sheet as a lower protective material (hereinafter this may be expressed as a back sheet).
Here the main component of the encapsulant includes ethylene-vinyl acetate copolymer (hereinafter this may be expressed as EVA), polyethylene (hereinafter this may be expressed as PE), ionomer (hereinafter this may be expressed as IO), polyvinyl butyral (hereinafter this may be expressed as PVB), etc.
Further, the encapsulant is essentially required to satisfy softness and impact resistance for protecting solar cell devices, to satisfy heat resistance for protecting solar cell modules in heat generation, transparency (total light transmission, etc.) for efficient arrival of sunlight to solar cell devices, as well as durability, dimensional stability, flame retardance, water vapor barrier property, etc. Moreover, the encapsulant is used generally in lamination, and therefore the aptitude thereof for lamination process and the appearance thereof after lamination are also important.
Here the lamination condition for solar cell modules may be set in many cases according to a trial-and-error method in consideration of various members to be used (encapsulant, back sheet, glass, wiring, cell, flux, etc.), and therefore there are some problems in that the condition setting takes much time and the cost of the members to be used is high.
One concrete problem with the appearance in condition setting is a phenomenon that protruding projections form on the face of the back sheet (hereinafter this may be expressed as projection phenomenon). Regarding this problem, PTL 1 says that, in vacuum lamination in producing solar cell modules by the use of a back sheet (back protective sheet for solar cell module), the back sheet may shrink and the solar cell device and the lead wire (tag) that connects the devices may follow the back sheet shrinkage, whereby the lead wire may deform and the distance between the solar cell devices may vary to cause the problem. As a measure against the problem, the reference discloses that the thermal shrinkage rate of the back sheet at 150° C. for 30 minutes is preferably at most 1.0%, more preferably at most 0.5%, even more preferably within a range of from 0.3% to 0.1% (see PTL 1, paragraph 0130).
On the other hand, PTL 2 discloses that a biaxially-oriented PET film has a large thermal shrinkage rate and therefore has some risks of wire (corresponding to lead wire in PTL 1) bending or cell misalignment especially in a process of producing large-size solar cell modules, and that for solving the problem of film shrinkage to be caused by the thermal shrinkage rate, a thermal fixation step for annealing the oriented film is needed in point of facilities, which, however, brings about film cost increase, or that is, there occurs another problem in point of cost in that inexpensive back sheets could not be obtained (see PTL 2, paragraph 0007). As the measure, the reference proposes a back sheet that contains a laminate of a polycarbonate film and a gas-barrier transparent deposition film (see PTL 2, Claim 1).